It can be abbreviated to

    SELECT: [num, name]
    FROM: [customers]
    WHERE EXISTS:
      SELECT: [name]
      FROM: [orders]
      WHERE:
        AND:
          - EQUALS: [customers.num, orders.customer_num]
          - LT: [price, 50]

It's not that different from formatted SQL. It's uglier, though, IMO, and more error prone. But to hate YAML for the fact that SQL looks less familiar in it, is a weird take.